An elderly women feeling so tired all the time... Intraoffice hematocrit is 35%. Peripheral blood smear shows many macrocytic red cells. On questioning, the woman, whose finances are limited, admits that has been living on a “tea and toast” type diet. She has been drinking a powdered orange juice substitute (Tang). She has not been taking vitamin pills because she feels she can’t afford them. A nutritional deficiency of which of the following is the most likely cause of this patient’s anemia?